Modular multilevel converters are often used because of their high efficiencies, their modularity and scalability, as well as for their ability to produce voltage waveforms with low harmonic content which effectively reduce the need for large alternating current (AC) filters. Several modular multilevel converter topologies exist, e.g. M2LC (also called MMLC and MMC) modular multilevel converter, in particular in high voltage direct current (HVDC) applications but also in motor drives etc.
The modular multilevel converter can be used as a converter interface for an energy storage systems (ESS), e.g. a Battery Energy Storage Systems (BESS), where a high voltage (Li-ion, Pb-Acid, NAS, SuperCapacitor or similar) direct current (DC) energy storage system is connected to a common DC link of the converter. However the very high voltage of the common DC link of the converter is a disadvantage in the ESS application, since the converter needs to have a correspondingly high voltage rating. The high voltage adds cost for insulation, as well as for fault handling and circuit breakers. The ESS can be equipped with a distributed DC breaker based on insulated-gate bipolar transistors (IGBTs), which adds a significant cost to the system, and produces losses.
Electrochemical batteries and supercapacitors will vary their voltage during a charge-discharge cycle. The discharge profile depends mainly on battery chemistry, but is also influenced by battery temperature, charge-discharge rate, aging of battery etc. Any kind BESS converter will need to handle the variable DC voltage of the battery(-ies), and that comes to an expense of de-rating (voltage-wise).
FIG. 1 shows typical voltage profiles for a battery discharged at different rates, wherein the C-rate is defined as current (in Amperes) divided with Coulomb capacity (in Ah).
FIG. 2 shows a standard BESS with a five-level M2LC (one phase leg shown) in which a plurality of half-bridge converter cells, each with a capacitor, are connected in series, four half-bridge cells forming the upper (positive) arm and four half-bridge cells forming the lower (negative) arm.
WO 2011/042050 discloses a voltage source converter with a first group of cells which is only capable of unipolar voltage contribution and a second group of cells being capable of bipolar voltage contribution. The circuit configuration allows for a higher flexibility in control.
WO 2012/103936 relates to eliminating a fault on a HVDC line by including at least one H-bridge submodule in a phase branch of a self-commutated converter.